Oxidation of alicyclic hydrocarbons



Patented June 9, 1942 oxmA'rroN F ALICYCLIC nxnaocmons Sumner II. McAllister, Lafayette, Cal-it, assignor to Shell Development Company, San Francisco Calif a corporation of Delaware No Drawing. Application February 29, 1940,

Serial No. 321,477

. 20 claims. (Cl. 260-533) This invention relates to a process for the production of aliphatic dicarboxylic acids, and it more particularly relates to the'economic production of aliphatic dicarboxylic acids by the oxidation of alicyclic hydrocarbons with nitric acid, in the presence or absence of oxidation-promoting catalysts, the operating conditions andparticularly the nitric acid concentration being controlled by a novel procedure and control mechanism so as to obtain high yields of the desired aliphatic dicarboxylic acids with a minimum consumption of the nitric acid,

It is known that aliphatic dicarboxylic acids may be prepared by the oxidation of various cyclic organic compounds. Such dicarboxylic acids have been previously produced by the oxidation of alicyclic organic compounds of the type of cyclobutane, cyclopentane, cyclohexane, alkylated cyclopentanes and cyclohexanes, as well as of cyclo-aliphatic alcohols and/or ketones, such as cyclobutanol, cyclopentanol, cyclopentanone, cyclohexanone, alkylated alicyclic alcohols and ketones, and the like.

It has also been known that the oxidation of these cyclic organic compounds, and particularly,

of the alicyclic hydrocarbons, in general requires the use of a relatively strong oxidizing agent, and that such cyclic compounds may be converted into aliphatic dicarboxylic acids by oxidation with nitric acid which may be considered to be one of the most suitable agents for this type of oxidation. Since nitric acid is a relatively expensive oxidizing agent, the economy of the nitric acid oxidation processes is primarily dependent upon the consumption of the nitric acid. In spite of the ready availability of a large number of cyclic organic compounds, such as the alicyclic hydrocarbons. alicyclic alcohols and alicyclic ketones, the nitric acid oxidation of these compounds has not, in the past, been considered economical. This is due mainly to the fact that, under the conditions of oxidation hitherto.usually employed, considerable quantities of dicarboxylic acids having a lesser number of carbon atoms than the material being oxidized were invariably produced, Also, when operating in accordance with the processes of the prior art, a substantial quantity of the employed nitric acid is reduced only to nitrogen peroxide (N204) It has nowbeen discovered that saturated and unsaturated alicyclic hydrocarbons which may or may not contain aliphatic side chains may be efliciently and economically subjected to nitric acid oxidation to produce high yields of aliphatic cyclohexanol,

carbon atoms per molecule as the alicyclic hydrocarbon subjected to oxidation. It has been further discovered that such eificient production of dicarboxylic acids may be obtained by effecting the oxidation reaction with nitric acid at a temperature within the range of from about 50 C. to

130 C. and preferably between about C. and C. while continuously maintaining the nitric acid concentration within the range between about 4% and 24% by continuously adding concentrated nitric acid thereto. In other words, the alicyclic hydrocarbons of'the above defined and hereinbelow more fully described class may be economically converted to .the corresponding aliphatic dicarboxylic acids by efiecting the nitric acid oxidation of the alicyclic hydrocarbons while continuously maintaining the dilute nitric acid solution within the described concentration range by the addition thereto of fortifying nitric acid. When efiecting the oxidation batchwise according to the present invention the nitric acid solution of the described concentration may be introduced into a reaction vessel and commingled therein with a quantity of the alicyclic hydrocarbon to be oxidized. The mixture is maintained within the above outlined temperature range, fortifying nitric acid being continuously added to maintain the desired concentration. When the present process is to be employed for the continuous production of the dicarboxylic acids, the aliphatic hydrocarbons may then be continuously fed into a reaction zone containing the nitric acid solution of the specified concentration maintained within the described temperature range, the nitric acid concentration being maintained substantially constant by the continuous addition thereto of the fortifying concentrated nitric acid. The reaction mixture, containin'g'the produced dicarboxylic acids, may be withdrawn from the reaction zone either continuously or otherwise depending on whether the process is to be efiected in a continuous or intermittent manner.

It has been further discovered that the nitric acid oxidation of the aliphatic hydrocarbons according to the present process results in a consumption of nitric acid which is less than that heretofore deemed necessary. This is due to the controlled'operating conditions which result in a material decrease in side reactions and in a reduction of a considerable proportion of the nitric acid to nitrous oxide and even to elemental nitrogen.

Representative alicyclic hydrocarbons which dicarboxylic acids having the same number of 55 y be effectively, emciently and economically oxidized with nitric acid in accordance with the process of the present invention are cyclobutane, cyclobutene, oyclopentane, cyclipentene, cyclohexane, cyclohexene, and the like, and their homologues and analogues, as well as the alkylated derivatives of these saturated and unsaturated alicyclic hydrocarbons, such as methyl cyclobutane, ethyl cyclobutane, methyl cyclopentane, methyl cyclohexane, l-methyl cyclopentene-2, l-methyl cyclohexene-2, l,l-dimethyl cyclopentane, 1,1,2-trimethyl cyclopentane, 1,1- .dimethyl cyclohexane, 1,3-dimethyl cyclohexane, and the like, and their homologues and analogues. As will be apparent to those skilled in the art, by a proper choice of the above and other similar saturated and/ or unsaturated alicyclic hydrocarbons, it is possible to produce a wide variety of aliphatic dicarboxylic acids. The unsaturated alicyclic hydrocarbons described herein are also known as cyclo-oleflns" (Hackhs Chemical Dictionary, 2nd ed.).

The most important single. factor which governs the consumption of nitric acid and the purity of the product obtained from the nitric acid oxidation of saturated and unsaturated alicyclic hydrocarbons is the concentration of the nitric acid in the reaction vessel. The temperature at which the nitric acid oxidation is eflected is a variable of somewhat less importance. Other factors which affect the economy of the process to some extent are the strength of the fortifying acid and the presence or absence of an oxidation-promoting catalyst.

The concentration of the nitric acid necessary for the eilicient oxidation of the defined alicyclic hydrocarbons to the corresponding aliphatic dicarboxylic acids having the same number of carbon atoms per molecule, is considerably lower than customarily employed for the oxidation of cyclic compounds. In general, the concentration of nitric acid is preferably maintained as' low as possible, consistent with a satisfactory reaction rate. In order that the reaction proceed at an appreciable rate using the preferred very low nitric acid concentration, the reaction is preferably executed at a somewhat high temperature. Generally, nitric acid concentrations ranging from about 8% to about 24% are preferred. However, concentrations as low as 4% may often be employed. Depending upon the concentration of the nitric acid maintained in the reaction zone, and somewhat upon the alicyclic hydrocarbons subjected to the oxidation therein, the temperature at which the oxidation is executed may rangefrom about 50 C. to 130 0., although temperatures between about 70 C. and 110 C. are preferred.

The yield and purity of the final product are somewhat dependent upon the concentration of the fortifying acid continuously added to maintain continuously the desired concentration of nitric acid in the reaction zone. Generally acid concentrations of about 50% to 70% HNO; have been found to be preferable to the more dilute acids which, however, may also be employed in some cases as the fortifying acid which is continuously introduced into the reaction zone to maintain therein a substantially uniform nitric acid concentration.

This process may be satisfactorily carried out with or without the aid of an oxidation-promoting catalyst. Any of the conventional oxidation catalysts such as the oxides and salts of the polyvalent metals are applicable and may be used if desired. The presence of a catalyst tends, in

general, other factors being equal, to increase somewhat the yield and throughout capacity, to

allow the use of somewhat lower temperatures,

hydrocarbon subjected to the nitric acid oxidation. Generally, the saturated alicyclic hydrocarbons require more severe oxidizing conditions than their unsaturated analogues. Furthermore, non-alkylated alicyclic hydrocarbons such as cyclopentane. cyclopentene, cyclohexane and the likerequire more severeoxidizing conditions, i. e. higher nitric acid concentrations and/or higher temperatures than their corresponding homologues containing alkyl side chains. The unsaturated alicyclic hydrocarbons containing relatively long side-chains are preferably oxidized under the most mild conditions, i. e. at relatively moderate temperatures and nitric acid concentrations near the lower limits of the described range.

Although the oxidation may be sfliciently realized at atmospheric pressures, somewhat higher pressures may also be employed. In fact, the use of super atmospheric pressures, which may be as high as 10 atm. or above, may be advantageous in that such pressures may allow the oxidation in a liquid state or phase and at higher temperatures, i. e. temperatures which are above the normalboiling point of the reaction mixture.

As stated, the process may be effected in a batch, intermittent, or continuous manner, it being understood that an essential feature of any apparatus for such oxidation is the provision of means whereby the concentration of the nitric acid in the reaction zone is maintained within the defined limits by the continuous introduction thereinto of relatively concentrated fortifying nitric acid. For instance, the apparatus may include a reaction vessel equipped with a stirrer, and containing dilute nitric acid of the desired concentration. This vessel may also contain means, such as a coil, for maintaining the desired reaction temperature. The alicyclic hydrocarbon may be continuously or otherwise introduced into this vessel from an outside storage tank, or the like. The fortifying, relatively concentrated nitric acid is also introduced into the reaction vessel at such a rate that the dilute nitricacid therein is maintained within the described desired range of concentration. The crude reaction mixture, which consists essentially of the desired aliphatic dica'rboxylic acid and dilutenitric acid, may be withdrawn continuously or intermittently, preferably from the bottom of the reaction vessel, and may then be treated to separate the dicarboxylic acid or acids from the dilute nitric acid. The fixed gases produced as a by-product of the oxidation reaction, may be withdrawn from the top of the reaction vessel, preferably after passage through a condenser which liquefles and returns into the reaction vessel any vapors of the unreacted alicyclic hydrocarbon and of the dilute nitric acid is continuously recirculated, as by means of a pump, through a reaction zone provided with packing 'banles and/or reaction-promoting catalyst. Such an apparatus may then be provided with means I near the bottom and the unreacted vapors with- I drawn with the gaseous reaction products 'from the top. The reaction may also be executed in a plurality of reactors connected in series or in a tubular reactor. The apparatus should, of

course, be resistant to hot dilute nitric acid. In

general, equipment having an acid-proof lining and equipment manufactured from corrosionresistant alloys, such as duriron, stainless steel, etc., are applicable.

The crude reaction product withdrawn from the reaction zone contains the desired aliphatic dicarboxylic acid in solution in dilute nitric acid along with any unoxidizedalicyclic hydrocarbon and small amounts of other products of side reactions. Although the desired dicarboxylic acid may be recovered from the crude reaction-mixture by any one of several methods, it has been found highly advantageous to first concentrate the solution to a pointwhere a substantial quantity of the desired dicarboxylic acid may be crystallized out upon cooling. This concentration may be most advantageously accomplished by conducting the crude reaction mixture, preferably without any pre-coolinggcontinuously into a suitable fractionating or stripping apparatus wherein a substantial portion of the water and nitric acid are removed. Such a process is advantageous since the last traces of unreacted hydrocarbons withdrawn with the reaction mixture undergo reaction during this flashing operation. Furthermore, it obviates the continuous loss of a small amount of unreacted alicyclic hydrocarbons which could not be economically recovered. By subjecting the hot crude reaction mixture to the mentioned substantially immediate flash distillation, the time that the dicarboxylic acids are in contact with hot nitric acid is kept at a controlled minimum. This helps tomaintain high yields and a pure product. The saving of heat afforded by this method is also of some commercial advantage. The dilute nitric acid solution recovered from such concentration step may be "easily reconcentrated and recycled.

, ther purified, if desired, by drying at C. to

50 0., preferably under decreased pressure, dissolving in a suitable solvent, such as acetone, decolorizing if desired, for instance, by means of charcoal (preferably degassed charcoal), and by finally recovering the pure dicarboxylic acid from the solvent solution. Obviously, other convenextraction, etc., may also be used. Since the mother liquor from the first crystallization may still contain appreciable quantities of aliphatic dicarboxylic acids, this solution, if desired, may

be further concentrated, and a second strike of' dicarboxylic acids may be crystallized out- The final liquor remaining after any number of concentrations and crystallizations, consists essentially of concentrated acid, but still contains traces of dicarboxylic acid. This final liquor may be reused in the process, for example, as the fortifying nitric acid added to maintain the desired nitric acid concentration in the reaction zone.

For the purpose of a better understanding of the present process, reference is made to the following example in which glutaric acid was produced by the nitric acid oxidation of cyclopentene. It is to be understood, however, that there is no intention of being limited by the described details of operation since numerous variations and modifications, which will be apparent to those skilled in the art, may be made within the scope of the appended claims.

Example Cyclopentene was introduced continuously at a rate of about 80 grams per hour per liter of reaction. mixture into a reaction vessel provided with a stirrer and containing a nitric acid solution the initial HNO: concentration of which was about 20%. Throughout the reaction, the mixture in the vessel was maintained at a tempera ture of about 110 C. and a pressure of about 10 atmospheres. In order to maintain the nitric acid concentration within the aforementioned desired range, a fortifying nitric acid of about 70% HNO: concentration was alsocontinuously fed into the reaction zone. However, due to a slightly lower rate of introduction of such fortifying acid, the nitric acid concentration in the reaction vessel was dropped gradually and was about 14.4% at the end of the run. Ananalysis of the reaction products indicated that there was a substantially complete conversion of the cy-v clopentene, the yield of dicarboxylic acids being about 74%. Substantially all of thisacid was glutaric acid, there being only small percentages of succinic acid present in the reaction mixture. The nitric acid consumption was about 4.5 moles per mole of dicarboxylic acid produced, thus indicating that the nitric acid consumption of this process is less than the consumption heretofore deemed necessary for the nitric acid oxidation of alicyclic hydrocarbons. The yield of the dicarboxylic acids may be further increased, thereby reducing the per mole consumption of nitric acid, by effecting the reaction at somewhat different conditions, as, for example, by lowerin somewhat the operating, temperature.

The above run was effected under a superat- N mospheric pressure in order to maintain the reactants in a liquid phase at the operating temperature. Obviously when other saturated and/- or unsaturated alicylic hydrocarbons are oxidized, and particularly when such oxidation is effected at lower temperatures, the operating pressure may be decreased, and, in fact, the reaction in some cases may be carried out even at atmospheric pressure.

The described process has distinct advantages over the previously known processes because of the low nitric acid consumption, and because of the high yield of the desired aliphatic dicartional methods of purification, such as solvent 75 boxylic acids having the same number of carbon atoms per molecule as the aiicyclic hydrocarbon subjected to the oxidation. Although the actual yield of such dicarboxylic acids from the aiicyclic hydrocarbons is somewhat lower than that obtained when aiicyclic alcohols and/or aiicyclic ketones are subjected to nitric acid oxidation under the described operating conditions, the ready availability and the low cost of the saturated and unsaturated aiicyclic hydrocarbons, and the fact that it is unnecessary to convert these aiicyclic hydrocarbons to the corresponding alcohols and/or ketones, renders the present process highly economical.

I claim as my invention:

1. A process for the production of glutaric acid which comprises continuously introducing cyclopentene into a nitric acid-containing solution maintained at a temperature of about 110 C., maintaining the concentration of said nitric acid a in said solution between about 14% and 20% by continuously adding fortifying nitric acid, continuously removing the crude reaction mixture and recovering glutaric acid therefrom.

2. The process according to claim 1 wherein the reaction is effected under a superatmospheric pressure thereby maintaining the reactants in a liquid state.

3. A process for the production of glutaric acid which comprises continuously introducing cyclopcntene into a nitric acid-containing solution maintained at a temperature between 50 C. and

130" C., maintaining the concentration of said nitric acid in said solution between about 8% and 24% by continuously introducing fortiiying v nitric acid as required, removing the crude reaction mixture and recovering glutaric acid therefrom.

4'. A process for the production of an aliphatic dicarboxylic acid which comprises continuously introducing a non-alkylated cycio-olefin into a nitric acid-containing solution maintained at a temperature between 70 C. and 110 C., maintaining the concentration of the nitric acid in said solution between about 4% and 24% by introducing iortiiying nitric acid as required, continuously removing the crude reaction mixture and recovering an aliphatic dicarboxylic acid therefrom.

5. The process according to claim 4 wherein the fortliying nitric acid is added continuously and wherein said acid is of a concentration of between about 50% and 70% HNOs.

6. The process according to claim 4 wherein the reaction is effected under a pressure sufficient to maintain the reactants, under the operating conditions, in a liquid state.

7. A process for the production of an aliphatic dicarboxylic acid which comprises continuously introducing a non-alkylated cyclo-olefin into a nitric acid solution maintained at a temperature above 50 C., maintaining the concentration of said nitric acid solution between 4% and 24% by introducing fortifying nitric acid as required, removing the crude reaction product and recovering the aliphatic dicarboxylic acid therefrom.

8. A process for the production of an aliphatic dicarbixylic acid which comprises continuously intrcducIng a non-alkylated aiicyclic hydrocarbon into a nitric acid solution maintained at a temperature above 50 C., maintaining the concentration. of said nitric acid solution between about 4% and 24% by introducing fortifying nitric acid as required, removing the crude reaction product and recovering the aliphatic dicarboxylic acid therefrom.

9. A process for the production of an aliphatic dicarboxylic acid which comprises reacting on aiicyclic hydrocarbon with a nitric acid solution maintained at a temperature between 70' C. and C., maintaining the concentration'of said nitric acid between 8% and 24% by introducing more concentrated nitric acid, removing the crude reaction mixture and concentrating the same by distillation, crystallizing aliphatic dicarbcxylic acid from the concentrated reaction mixture and finally purifying the aliphatic dicarboxylic acid by treatment with degassed charcoal.

10. In a process for the production of an allphatic dicarboxylic acid which includes the introduction oi an aiicyclic hydrocarbon into a nitric acid-containing solution maintained at a temperature between 70 C. and 110 C., the step oi maintaining the concentration of the nitric acid in said solution between about 4% and 24% substantially throughout the reaction between the nitric acid and the aiicyclic hydrocarbon by the addition of fortifying nitric acid, whereby nitric acid is consumed economically.

11. In a process for the production of an aliphatic dicarboxylic acid, the stepsof reacting a cycle-olefin in the presence of an oxidationpromoting catalyst with a nitric acid solution maintained at a temperature between about 70 C. and 110 C., maintaining the concentration of said nitric acid solution between about 8% and 24% by introducing more concentrated nitric acid, removing the crude reaction mixture, concentrating said mixture by distillation, and recovering the dicarboxylic acid from said concentrated reaction mixture.

12. The process according to claim 11 wherein aliphatic dicarboxylic acid is recovered from the concentrated reaction mixture by crystallization.

13. In a process for the production of an allphatic dicarboxylic acid, the steps of reacting an aiicyclic hydrocarbon in the presence of an oxidation-promoting catalyst with a nitric acid solution maintained at a temperature between about 70 C. and 110 C., maintaining the concentration of said nitric acid solution between about 8% and 24% by continuously introducing more concentrated nitric acid, removing the crude reaction mixlure and recovering the allphatic dicarboxylic acid therefrom.

14. In a process for the production of an aliphatic dicarboxylic acid, the steps of reacting an aiicyclic hydrocarbon in the presence of an oxidation-promoting catalyst with a nitric acid solution maintained at a temperature above 50 C.,

maintaining the concentration of said nitric acid solution between about 8% and 24% by introducing more concentrated nitric acid, removing the crude reaction mixture and recovering the aliphatic dicarboxylic acid therefrom.

15. In a process for the production oi an allphatic dicarboxylic acid, the steps of reacting an aiicyclic hydrocarbon in the presence of an oxidation-promoting catalyst with a nitric acid solution maintained at a temperature of above 50 C., maintaining the concentration of said nitric acid solution between about 4% and 24% by introducing more concentrated nitric acid, removing the crude reaction mixture and recovering the aliphatic dicarboxylic acid therefrom.

16. In a process for the production of an aliphatic dicarboxylic acid, the steps of reacting an aiicyclic hydrocarbon with nitric acid maintained at a temperature of above 50 C., maintaining the concentration of said nitric acid soacid solution maintained between 4% and 24% nitric acid concentration and at a temperature of between about 50 C. and 130 C.

19. In a process for the production of an aliphatic dicarboxylic acid, the steps of forming a body of reaction solution containing dilute nitric acid in a reaction zone, continuously introducing an alicyclic hydrocarbon into said body of reaction solution to form the aliphatic dicarboxylic acid, continuously introducing a concentrated fortifying nitric acid into said reaction zone to maintain the concentration of the dilute nitric acid in said zone substantially constant in the range of from 4% to 24%, and continuously withdrawing the reaction mixture comprising dilute nitric acid and the dicarboxylic acid from said reaction zone at a rate correlated to the rate of additions to said zone so as to maintain a substantially constant body of reaction solution in said zone. v

20. In a process for the production of an aliphatic dicarboxylic acid, the steps of forming a body of reaction solution containing dilute nitric acid in a reaction zone, feeding an alicyclic hydrocarbon to said body of reaction solution, introducing a fortifying nitric acid into said reaction zone at a rate sumcient to maintain oxidizing conditions and dilute nitric acid having a concentration in the range of from 4% to 24% in said reaction zone, and withdrawing the reaction mixture containing formed aliphatic dicarboxylic acid from said reaction zone at a rate correlated to the rate of additions to said zone so as to maintain a body of reaction solution in said zone.

SUMNER H. McALLISTER. 

